Supplementary inflatable restraints (air bags) in automobiles typically utilize an acceleration sensor element located in the engine compartment. If the sensor element is of the mechanical accelerometer type, it is mounted in an open container along with a printed circuit board and other components, all of which are surrounded by a potting material which protects the sensor element and other components from the hostile environment of the engine compartment. As such, the sensor assembly is subjected to extreme thermally-induced stresses resulting from thermal cycling and thermal shock. In addition, unless the sealing integrity of the potting material is continuously maintained, the sensor assembly is also subject to water, salt and other chemicals which could have a deleterious effect on the sensor's performance. Thus the reliability of the sensor element depends on the ability of the potting material to prevent the intrusion of harmful substances.
While the above considerations are generally true of any electrical assembly exposed to such a harsh environment, an additional requirement of the potting materials used to encapsulate an acceleration sensor element is that the material must faithfully and reliably transmit the acceleration of the vehicle-mounted container to the sensor element within the container. Conventionally, a potting material which is able to fully transmit the acceleration of its container to the sensor element is said to have a transfer function of one with respect to acceleration.
A known potting material for acceleration sensor elements is composed of methylene di-p-phenylene isocyanate (MDI) and castor oil based urethane, and is implemented using "pot-on-sand" (POS) processing techniques. Such techniques involve pouring a mixed urethane over hot sand contained in the sensor container, or by vibrating hot sand into the urethane in the sensor container. Once cured, the potting material provides environmental protection for the sensor element. However, such potting materials have a number of disadvantages. Notably, they are extremely difficult to process, and any processing error may result in an assembly having diminished reliability. Even if correctly processed, the sensor assemblies at times do not pass rigorous environmental testing for water intrusion into the sensing element following salt spray or thermal cycling tests. In addition, a conformal coating over the printed circuit board is typically required to protect the board from the sand in the potting material, resulting in added processing steps and costs. Finally, because vibration is required to evenly distribute the sand in the urethane, some fixturing, adhesive or otherwise, must be used to hold the sensor element in place, which adds additional costs to the manufacture of the acceleration sensor assembly.
As a solution, U.S. Pat. No. 5,185,498 to Sanftleben et al. teaches a potting compound which is formulated to meet or exceed the mechanical, physical and material requirements for use in a potted acceleration sensor assembly used in an automotive environment, yet avoids the processing shortcomings noted above for prior art potting materials. Specifically, Sanftleben et al. teach a potting compound composed of polybutadiene urethane having physical and mechanical properties which enable the potting compound to maintain its sealing integrity under intense thermal cycling conditions to exclude foreign elements from the sensor element. Though the polybutadiene urethane potting compound taught by Sanftleben et al. is a rubbery elastomeric material, it is unexpectedly capable of faithfully transmitting acceleration from its container to the sensor element, so as to have a transfer function of nearly one with respect to acceleration.
Furthermore, Sanftleben et al. teach that additions of a novolac epoxy resin to the potting compound of up to about 10 weight percent enhances the ability of the potting compound to adhere to the interior walls of the container, so as to further reduce the likelihood of water intrusion into the assembly. Sanftleben et al. disclose the use of a metal container in which the sensor assembly is potted. In general, a potting material can typically adhere to a metal or a metal coated with an electrically deposited epoxy-based paint (E-coated metal) more readily than to engineering resins, such as polybutylene terephthalate (PBT) and polyethylene terephthalate (PET). Consequently, it would be desirable if further improvements in the adhesion properties of the potting compound taught by Sanftleben et al. could be achieved in order to promote the reliability of the sensor element when housed within a container formed from such engineering resins. Furthermore, it would also be desirable if the adhesion characteristics of the potting compound were sufficient so as to enable the potting compound to be used as an adhesive, coating, or encapsulating material for a wide variety of applications.